In order to obtain accurate readings from a flowmeter, it must be periodically calibrated by determining its characteristic, or K-factor, the K-factor is the constant of proportionality between the flow rate of the fluid flowing through the flowmeter and the response given by the flowmeter. A typical turbine-type flowmeter develops electrical oscillations proportional in number to the volume of flow through the flowmeter. In this case, the characteristic is expressed in terms of the number of pulses generated by the flowmeter per unit volume of fluid passing through the flowmeter. The flowmeter characteristic is a function of the type of fluid, as well as the fluid temperature, pressure, and flow rate, and varies as the flowmeter parts wear in the course of use. An apparatus to determine the characteristic of a flowmeter while in an operating fluid system is called a prover. An apparatus to determine the characteristic of a flowmeter on a test stand, i.e., not in a fluid system, is called a calibrator.
It is well known to determine the characteristic of a flowmeter by comparing its response to a ballistic flow calibrator or prover connected in series with the flowmeter. A ballistic prover uses a piston that travels in a cylindrical chamber in synchronism with the fluid traveling through the flowmeter. By measuring the time interval required for the piston to travel through a known volume of the chamber, an average flow rate can be calculated. This is then used to determine the flowmeter characteristic. U.S. Pat. No. 3,492,856 discloses a ballistic flow calibrator in which the piston has a passage through it. A valve seals the passage when it is closed and permits fluid flow through the piston when it is opened. U.S. Pat. No. 4,152,922 discloses a ballistic flow calibrator, with an auxiliary piston and an auxiliary cylinder, to control a fluid displacement measuring piston which moves through a fluid displacement measuring cylinder as a fluid barrier the same distance as the auxiliary piston moves through the auxiliary cylinder.
In existing provers the release and return of the piston involves many difficult mechanical problems which have not been readily overcome. Mechanisms tend to be complex and the prover itself bulky and costly to construct. Some provers utilize complex reversing valves to reverse the direction of flow in the cylinder and thereby return the piston to its original position. Other designs utilize devices to retract the piston and restrain it in the upstream position or bypass the flow of the piston by means of a poppet or bypass valve when the prover is not being used in a proving test. Provers utilizing valves to reverse the direction of flow are known as bidirectional provers because the proving test may be made with the piston traveling in either direction. Provers utilizing devices to retract and restrain the piston are known as unidirectional provers because the fluid and the piston always travel in the same direction in the cylinder during a test.
In U.S. Pat. No. 3,492,856 issued on Feb. 3, 1970 to Francisco, a unidirectional flow meter calibrating apparatus is disclosed employing a piston within a conduit, where the piston is retrained in the upstream position by means of a complex motor, clutch and cable assembly located upstream of the conduit. A poppet valve, held open by the cable, provides a fluid passage through the piston when the apparatus is not being used for flow measurements. Releasing the cable permits fluid pressure to close the poppet valve setting the piston in motion.
U.S. Pat. No. 4,152,922, issued May 8, 1979, to Francisco discloses a prover in which a measuring piston is returned and restrained in its upstream position by means of a second, control piston. The control piston travels through a separate control cylinder and is linked to the measuring piston by a rod. A source of pressurized air is used to move of the control piston. In U.S. Pat. No. 4,794,785, issued on Jan. 3, 1989, to Cohrs et al., a similar prover is disclosed wherein the control cylinder is moved by pressurized hydraulic fluid.
In summary, the mechanisms used in prior art provers to return the measuring piston to the upstream position have tended to be complex, costly to manufacture and have often required expensive, external hydraulic or pneumatic propelling apparatus.